1. Field of the Invention
The present invention relates to an electrical device securing the normality of its output signals, particularly to such a system using the electrical device suitable for composing an input and output terminal or a transmittal terminal of a high reliability system as a fault tolerant system.
2. Description of the Related Art
The first conventional method for constructing a fail safe system is popularly realized by detecting faults in an electrical device or an electrical circuit adapting a self-checking methodology and realizing continuous operations of the fail safe system by changing the configuration of the system based on the fault detecting results.
Examples of such an approach are disclosed in the following references.
(1) "A 32-bit Fail Safe MPU Board" by Masayuki Umeyama,
Japan Signal Technique Report, Vol. 16, No. 3, pp. 11-16 (1992)
(2) "Development of A 32-bit Fail Safe CPU" by Noboru Asano, Traffic Electronics and Rail Way Research Committee Report of The Institute of Electrical Engineer of Japan, TER-92-35, pp. 31-41 (1992).
The disclosed methods for realizing a fail safe function detect faults in circuits by using a duplicated circuit such as a memory or a micro-processor and collating signals on buses connected to the duplicated circuit.
Then, as in the second reference, a method of feeding back output signals and collating the output signals and signals to be output, is popularly used to secure the normality of the output signals.
The above-mentioned conventional techniques disclosed in the Japan Signal Technique Report and the Traffic Electronics and Rail Way Research Committee Report of The Institute of Electrical Engineer of Japan, are effectual in the points that those techniques can be easily realized by using general purpose micro-processors or memories. However, the convenience of connection with an outer circuit is not much regarded in the techniques. In the above-mentioned techniques, it is required to use a duplex circuit as a circuit to be connected to a self-checking circuit. However, although a circuit synchronizing with a processor, such as a memory, can be simply duplicated and dealt with logically as a simple circuit, such a circuit operating asynchronously with a processor as a transmission interface connected with an outer circuit disturbs the synchronization of a whole system, only by simply duplicating the circuit. Then, as for such a circuit, it is desirable to provide two logically different circuits or devices, but it is not always necessary to connect such a circuit to an outer duplex bus as shown FIG. 7. And many of conventional transmission interfaces are designed so as to be connected to a non duplex bus (hereinafter called as a simplex bus) and can not be connected to a duplex bus as shown FIG. 7. Then, since error correcting detection of output signals is possible if error correcting codes are added to signals output to a bus, duplication of an outer bus is not always necessary.
The conventional output collating method of feeding back output signals is an excellent one for securing the normality of output signals. And, particularly, the method can detects a fault such as disconnection of a wire, a contact failure, etc., as to output signals of a circuit on a wired board, and a fault such as an inferior wire-bonding, an inferior soldering of a pin (or a lead), etc., as to a circuit in a semiconductor package. However, in the method, a counter-measure to increasing of the required points of contact or terminals of pins (or leads) is not much regarded yet. Then, the same number of terminals for the feedback as that for output signals is needed for feeding back the output signals. For example, totally terminals are necessary in the method for an address signal of 32 bits and a data signal of 32 bits. Further, if an outer bus is duplicated as mentioned above, 256 terminals are necessary. Especially, in such a system, an increasing of terminals in a semiconductor device causes an increase of a chip size or a package size which causes in its turn an increase of a failure rate besides weight and volume of a device.
Further, since a use of an electrical device having a self-checking function is restricted to a special field and the mass production effects are not large, a cost of a device becomes very high if design and production is individually done for each device for adapting the device to a simplex bus or a duplex bus.